| Objective:To investigate the changes of auditory threshold,ribbon synapses and mitochondria oxidative stress,inflammatory response in inner hair cell regions of C57BL/6J mice(3 weeks old immature mice,2 months old young mice,6 months old senior mice)after exposing to intense white noise of 110 dB SPL for 2 hours.Then we observe the damage caused by noise exposure in mice of different ages and explore the pathological mechanism of cochlear injury.Methods:(1)Detection of mice hearing threshold using subjective auditory brainstem response technique(ABR)(2)Using whole mount preparation and immunofluorescence technique to observe the number of presynaptic specific protein CtBP2,postsynaptic specific receptor GluA2 and colocolized punctain cochlear in inner hair cells by confocal microscopy(3)At the same time,the degree of damage of mitochondria was evaluated by specific oxidative damage product 8-OHdG.(4)Using the frozen section technique and immunofluorescence staining technique,the inflammatory response of the cochlea was evaluated by staining the inflammatory factor caspase1.(5)16 hearing normal 3-week-old C57BL/6J mice were randomly divided into 4 groups,3 of them are experimental groups,they were the 1-day measurement group after noise treatment(P1)and the 7-day measurement group after noise treatment(P7),the 14-day measurement group(P14)after noise treatment,and the remaining 1 group was a blank control group.4 mice per group(8 cochlea).The mice in the experimental group were exposed to 110 dB SPL noise for 2 hours,and ABR thresholds were detected after 1 day,7 days,and 14 days noise exposure,respectively.Result:(1)Audiological results:the hearing changes after noise exposure in mice of 3weeks old and 2 months old were similar,that is,the threshold threshold of all frequencies increased on the first day after exposure(p<0.05),but the threshold of high-frequency hearing increased significantly,while the threshold of low-frequencyhearing increased limited.On the 7th day,except for several high-frequency(24K and32K),most of the frequency thresholds returned to normal.On the 14 th day,all the frequency thresholds had no statistical difference with the control.However,on the first day after noise exposure,all frequency thresholds of6-month-old mice increased significantly,and the threshold of low-frequency also increased significantly(p < 0.01).On the 7th day,only part of the frequency threshold was restored(16K),and on the 14 th day,part of the frequency threshold was not restored(click、8K and 24K)(p < 0.05),but the difference was smaller than that on the 7th day.(2)Ribbon synaptic results:the number of synapses in 3-week-old and2-month-old mice was similar after noise exposure,that is,the number of CtBP2 and GluA2 decreased immediately on the first day after noise exposure,and the co-localization of the two groups also decreased(p < 0.01).On the 7th day,the number of synapses recovered,but there were still significant differences between the two groups(p < 0.05).On the 14 th day,the number of synapses returned to normal(P > 0.05).However,on the first day after noise exposure,the number of CtBP2,GluA2 and co-localization decreased significantly in 6-month-old mice(p < 0.01).On the 7th day,the number of synapses partially recovered,but there was still a significant statistical difference(p < 0.01).On the 14 th day,the number of synapses further recovered,but there was still a significant difference compared with the control group(P < 0.05).(3)Oxidative stress results: the oxidative stress injury of 3-week-old and2-month-old mice after noise exposure was similar.On the 1st and 7th day after noise exposure,8-OHdG in the cytoplasm of inner and outer hair cells was highly expression,and 8-OHdG was more expressed in inner hair cells than in outer hair cells.On the 14 th day,a small amount of 8-OHdG expression was observed in the above regions.However,on the 1st and 7th day after noise exposure,the level of 8-OHdG in6-month-old mice was similar to that in 3-week-old and 2-month-old mice,but on the14 th day,the expression level of 8-OHdG near inner and outer hair cells was more significant than that in 3-week-old and 2-month-old mice.There was no significant loss of hair cells and cilia in the middle turn of basilar membrane regions of the 2 month old and 6 month old mice after noise exposure.(4)Inflammatory factor result: on days 1 and 7 after noise exposure,caspase 1was expressed in the basilar membrane,vascular striate and spiral ganglion and snail axis.On the 14 th day,the immunofluorescence of the above region returned to the control level.However,on days 1 and 7 after noise exposure,the 6-month-old mice showed significant expression of caspase1 in the above region,similar to 2 months old mice,but on day 14,there was still a small amount of caspase1 expression in the above region.Conclusion:(1)The changes of hearing and ribbon synapse were similar after noise exposure in 2 months old and 3 weeks old mice.Six months old mice were more sensitive to noise exposure,and their hearing changes and synaptic changes were more significant.(2)Compared with 2 months old mice,the 8-OHdG content of DNA oxidative stress products in the cochlea was similar on the 1st and 7th day after noise exposure,and the 14 th day,8-OHdG in the cochlea of 6 months old mice were still high,which may be the reason why hearing loss persists.(3)Compared with 2 months old mice,the inflammatory factor caspase1 in the cochlea was similar on the 1st and 7th day after noise exposure,and the 14 th day,caspase1 in the cochlea of 6 months old mice were still high,which also may be the reason why hearing loss persists. |
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